The present invention relates to optical heads used for CD or DVD players.
In recent years Digital Versatile Disc (DVD) or originally "Digital Video Disc" technology has been promoted as the next revolutionary consumer electronic product to replace the popular compact disc (CD) players. However, DVD players are not widely accepted and are only manufactured in relatively small quantities by major Japanese companies. One of the difficulties in manufacturing the DVD players is in designing an optical pickup which can read both the new DVD formatted discs and the old CD discs.
FIG. 1 illustrates this difficulty with the optical pickup design. A laser source 110 emits a beam of light which is reflected by a beamsplitter 130 to a collimating lens 140 and then focused by the objective lens 150 on the disc medium 160. Because the laser beam is focused through the medium substrate to its back surface 161 the objective lens is designed to correct for the spherical aberration induced by the substrate thickness. In another words, the medium substrate forms an integral part of the objective lens. The objective lens can not focus to a diffraction limited spot if a different thickness is used for the substrate.
The light reflected back by the back surface of the medium after passing through the two lenses is then focused on the detector 170 through the beamsplitter 130. The astigmatism created by the thickness of the beamsplitter in the returning beam is used to produce the focus error servo signal. In most CD players the optical pickup also contains a grating 120 to split the laser beam to produce three focused beams on the medium to provide the tracking error servo signal.
There is no difference in the optical principle for the optical pickup for CD players and DVD players. However, because the DVD format uses a track spacing of 0.84 .mu.m and smaller pit size, the laser spot size required on the medium surface to read the data has to be smaller than 0.55 .mu.m. As a result, a higher NA objective lens is used in the optical pickup for DVD players. With such a high NA objective lens, the diameter of the focused beam spot on the medium is sensitive to the tilting of the medium substrate. Hence, a thinner substrate is selected for the DVD disc. Referring back to the optical system in FIG. 1, a DVD optical pickup can be made by choosing a proper objective lens which is designed for the 0.6 mm substrate thickness for the DVD disc and a laser diode with 650 nm wavelength to achieve the spot size needed to read the data on the DVD disc. The following Table lists the differences between the DVD pickups and the CD pickups:
TABLE 1 ______________________________________ CD DVD OBJECTIVE LENS NA = 0.45 NA = 0.6 ______________________________________ SUBSTRATE THICKNESS 1.2 mm 0.6 mm LASER WAVELENGTH 780 nm 635-650 nm TRACK PITCH 1.6 .mu.m 0.84 .mu.m ______________________________________
As can be seen in the Table above, there are three major differences between the CD system and the DVD system. The first one is the medium thickness. The DVD lens is designed to correct the spherical aberration caused by the 0.6 mm substrate. When a CD disc with its 1.2 mm thickness substrate is put in a DVD player with the DVD objective lens, the focused beam spot is severely aberrated.
The second difference is due to the difference in the track pitch between the DVD disc and the CD disc. A three beam method is commonly used in the CD optical pickup. This method requires the two outside beams to be separated by half of the track spacing. Since the CD disc has a track pitch of 1.6 .mu.m and the DVD disc has a track pitch of 0.84 .mu.m, the two outside beams for the three beam tracking can only be set for one disc type and not the other.
The third difference is the wavelength of the laser used for the CD pickup and the DVD pickup. This is caused by the dye used in the CD-R discs used by the CD recordable players. The particular dye used in most of the CD-R discs has a peak absorption at 650 nm. As a result, when a CD-R disc is inserted into a DVD player with only the 650 nm laser, most of the light incident on the medium is absorbed by the dye and little light is reflected back to the detector.
FIG. 2 shows one of the current DVD optical pickup designs which solves all three problems above. As shown, the laser beam emitted by a 780 nm laser diode 210 is combined with the laser beam emitted by a 650 nm laser diode 220 through the use of a beamsplitter 230. A three beam grating 290 is located in front of the laser 210. The remaining optical system is the same as in FIG. 1. Both beams can be individually reflected by a second beamsplitter 240. After passing through the collimating lens 250, the beams are focused by an objective lens 260 on a medium 270. The reflected beams are focused by the lenses 260 and 250 through the beamsplitter 240 to a detector 280. When a DVD disc is inserted into a player containing this optical pickup, the 650 nm laser diode 220 is turned on. However, when a CD disc is inserted, the 780 nm laser diode is turned on and the laser beam is focused on the surface 272. Since the objective lens 260 is designed for correcting for the substrate thickness of the DVD disc, the 780 nm beam on the medium will be aberrated. To minimize the aberration, an aperture 292 can be placed in front of the 780 nm laser diode 210 to limit the beam incident on the objective lens 260 to a smaller numerical aperture. It is also possible to implement a variable aperture or wavelength sensitive aperture 294 placed in front of the objective lens 260.
In FIG. 2 a three beam grating 290 is shown in front of the 780 nm laser diode 210. Therefore, in CD mode of operation, the three beam tracking method can be used. In the DVD mode of operation a single beam tracking method is commonly used. However, in this design a three beam grating 296 can be placed in front of the 650 nm laser diode 220 so that three beam tracking can also be implemented for the DVD mode of operation. There are two difficulties with this design. The major one is the uncorrected aberration in the 780 nm beam due to the substrate thickness. The second is the efficiency of the beamsplitter 230. Unless the beamsplitter 230 is an expensive polarization beamsplitter, half of the light emitted by either laser diode 210, or 220, is lost.